This invention relates to a process for treating a feed gas. In particular, the invention relates to a process for removing or at least reducing the level of carbon dioxide and water in a feed gas to render it suitable for downstream processing. The invention is especially useful in removing carbon dioxide and water from air which is to be employed as a feed gas in a process for the cryogenic separation or purification of air.
Carbon dioxide is a relatively high boiling gaseous material and removal of this and other high boiling materials for example water which may be present in a feed gas is necessary where the mixture is to be subsequently treated in a low temperature, for example cryogenic, process. If relatively high boiling materials are not removed, they may liquefy or solidify in subsequent processing and lead to pressure drops and flow difficulties in the downstream process. It may also be necessary or desirable to remove hazardous, for instance explosive materials prior to further processing of the feed gas so as to reduce the risk of build-up in the subsequent process thereby presenting an explosion hazard. Hydrocarbon gases, for example acetylene, may present such a hazard.
Several methods are known for removing carbon dioxide and water from a feed gas by adsorption on to a solid adsorbent including temperature swing adsorption (TSA) and pressure swing adsorption (PSA), thermal pressure swing adsorption (TPSA) and thermally enhanced pressure swing adsorption (TEPSA).
Generally, in these processes water and carbon dioxide are removed from a feed gas by contacting the mixture with one or more adsorbents which adsorb water and carbon dioxide. The water adsorbent material may be for example silica gel, alumina or a molecular sieve and the carbon dioxide adsorbent material may typically be a molecular sieve, for example a zeolite. It is conventional to remove water first and then carbon dioxide by passing the feed gas through a single adsorbent layer or separate layers of adsorbent selected for preferential adsorption of water and carbon dioxide in a column. Removal of carbon dioxide and other high boiling components to a very low level is especially desirable for the efficient operation of downstream processes.
After adsorption, the flow of feed gas is shut off from the adsorbent bed and the adsorbent is exposed to a flow of regeneration gas which strips the adsorbed materials for example carbon dioxide and water from the adsorbent and so regenerates it for further use.
In a TSA process for carbon dioxide and water removal, atmospheric air is typically compressed using a main air compressor (MAC) followed by water-cooling and removal of the thus condensed water in a separator. The air may be further cooled using for example refrigerated ethylene glycol. The bulk of the water is removed in this step by condensation and separation of the condensate. The gas is then passed to a molecular sieve bed or mixed alumina/molecular sieve bed system where the remaining water and carbon dioxide are removed by adsorption. By using two adsorbent beds in a parallel arrangement, one may be operated for adsorption while the other is being regenerated and their roles are periodically reversed in the operating cycle. In this case, the adsorbent beds are operated in a thermal swing mode with equal periods being devoted to adsorption and to regeneration.
As the component which is being removed from the feed gas is adsorbed while the bed is on-line the adsorption process will generate heat of adsorption causing a heat pulse to progress downstream through the adsorbent. The heat pulse is allowed to proceed out of the downstream end of the adsorbent bed during the feed or on-line period. During the regeneration process, heat must be supplied to desorb the gas component which has been adsorbed on the bed. In the regeneration step, part of the product gas, for instance nitrogen or a waste stream from a downstream process, is used to desorb the adsorbed components and may be compressed in addition to being heated. The hot gas is passed through the bed being regenerated so removing the adsorbed carbon dioxide and/or water. Regeneration conventionally is carried out in a direction counter to that of the adsorption step.
In a PSA system, cycle times are usually shorter than in a TSA system, but feed temperature and pressure and the regeneration gas often are similar. However in PSA systems, the pressure of the regeneration gas is lower than that of the feed gas and the change in pressure is used to remove the carbon dioxide and water from the adsorbent. Regeneration is suitably commenced before the heat pulse mentioned above in relation to TSA has reached the downstream end of the bed. The direction of the heat pulse is reversed by the process of regeneration and the heat which derived from the adsorption of the gas component in question is retained in the bed and used for desorbing that component during regeneration. In contrast to TSA one thus avoids having to heat the regeneration gas.
Thermal pressure swing adsorption (TPSA) is also suitable for removing carbon dioxide and water from the feed gas. In a TPSA system water is typically confined to a zone in which a water adsorption medium is disposed for example activated alumina or silica gel. A separate layer comprising a molecular sieve for the adsorption of carbon dioxide is typically employed and the molecular sieve layer and the zone for adsorption of water conventionally are separate. By contrast with a TSA system water does not enter the molecular sieve layer to any significant extent which advantageously avoids the need to input a large amount of energy in order to desorb the water from the molecular sieve layer. A TPSA process is described in U.S. Pat. No. 5,885,650 and U.S. Pat. No. 5,846,295.
Thermally enhanced PSA (TEPSA), like TPSA, utilises a two stage regeneration process in which carbon dioxide previously adsorbed is desorbed by TSA and adsorbed water is desorbed by PSA. In this process, desorption occurs by feeding a regeneration gas at a pressure lower than the feed stream and a temperature greater than the feed stream and subsequently replacing the hot regeneration gas by a cold regeneration gas. The heated regenerating gas allows the cycle time to be extended as compared to that of a PSA system so reducing switch losses as heat generated by adsorption within the bed may be replaced in part by the heat from the hot regeneration gas. A TEPSA process is described in U.S. Pat. No. 5,614,000.
In contrast to PSA, TSA, TEPSA and TPSA all require the input of thermal energy by means of heating the regeneration gas but each procedure has its own characteristic advantages and disadvantages. The temperatures needed for the regenerating gas are typically sufficiently high, for example 100xc2x0 C. to 200xc2x0 C., as to place demands on the system engineering which increases costs. Typically, there will be more than one unwanted gas component which is removed in the process and generally one or more of these components will adsorb strongly for example water, and another much more weakly for example carbon dioxide. The high temperature used for regenerating needs to be sufficient for the desorption of the more strongly adsorbed component.
The high temperature employed in a TSA, TPSA and TEPSA systems may require the use of insulated vessels, a regeneration gas preheater and an inlet end precooler and generally the high temperatures impose a more stringent and costly mechanical specification for the system. In operation, there is extra energy cost associated with using the purge preheater.
The PSA system avoids many of these disadvantages by avoiding the need for coping with high temperatures, although the short cycle time which characterises PSA brings its own disadvantages.
EP-A-925821 describes a method for operation of a pressure swing adsorber in a PSA process air pre-purifier which takes into account inlet air conditions. The object of the invention in EP-A-925821 is to provide an improved method for controlling the cycle time of a PSA air pre-purifier and continuous control is exerted depending on parameters of inlet air feed. Air feed conditions are monitored to ascertain the moisture content of air being fed into the adsorber. EP-A-925821 is not concerned with TSA, TPSA or TEPSA processes.
In designing a TSA, TEPSA or TPSA system, one conventionally takes account of ambient prevailing conditions in the locality in which the process is to be operated as the level of water in the feed gas changes according to variations in local temperature and relative humidity. These factors vary on a continuous basis and daily or seasonal differences may be high and hence the level of water in the feed gas may vary considerably. Conventionally, operating parameters in TSA, TEPSA and TPSA processes have been selected to take account of the most adverse ambient conditions likely to be encountered to ensure efficient operation of the process. The process conditions are pre-selected and remain constant during operation in order to ensure that the feed gas having the highest likely content of water may be processed without risk of exceeding the capacity of the system to remove water and so avoiding water being passed to a downstream process.
The inventors have now found that TSA, TEPSA and TPSA systems need not be operated under constant conditions sufficient to cope with the most adverse ambient conditions likely to be encountered as is presently the norm but, surprisingly, the process operating conditions may be varied according to fluctuations in the ambient conditions by measuring one or more parameters relating to the composition of the feed gas to provide major energy savings yet still operate the process efficiently.